The invention relates to a low-pressure gas discharge lamp comprising
a tubular lamp vessel with two ends, which lamp vessel is sealed in a gastight manner at both ends over current supply conductors which extend from the exterior of the lamp vessel through respective ends to respective adjacent electrodes arranged in the lamp vessel;
a lamp cap which defines an axis, which is connected to the end of the lamp vessel, and which is provided with a contact pin which is connected to at least one current supply conductor;
an adapter which defines an adapter axis substantially coinciding with the axis of the lamp cap, which adapter has a bottom extending transversely to the adapter axis and an upright circumferential wall surrounding the adapter axis, whereby a cavity is realized in the adapter, inside which cavity the adapter is provided with a contact element comprising a retaining element by means of which the contact element is electrically connected to the contact pin and is mechanically retained on the contact pin of the lamp cap.
Such a low-pressure gas discharge lamp is known from U.S. Pat. No. 4,994,710. The known lamp is a low-pressure mercury vapor discharge lamp which has coating of fluorescent powder on the inner surface of the discharge vessel. The fastening of the adapter to the lamp vessel of the known lamp is realized by means of the contact element and by means of an external contact point. For this purpose, the contact element is fastened to the contact pin of the lamp by means of a retaining element comprising a scraping contact on the one hand, and on the other hand is fastened to the external contact point by means of rolling over, whereby a flange is formed. The external contact point extends from the cavity through the bottom to the exterior of the adapter and has a widened portion at the area where it issues from the adapter. The flange and the widened portion together fix the adapter on the external contact point. The known lamp has the disadvantage that an external contact point is necessary for fixing the adapter on the lamp. A widened portion is to be formed in the contact point for fixing the adapter to the contact point, and this portion is to be rolled over on the contact element, which is a comparatively expensive and laborious process. Another disadvantage of the lamp is that a lamp provided with a xe2x80x9cblind adapterxe2x80x9d, i.e. an adapter without an external contact point, is not easily realizable in this way.
The invention has for its object to provide a lamp of the kind described in the opening paragraph whose construction is simple and easy to realize, thus rendering possible a simple electrical connection at one end of the lamp.
According to the invention, this object is achieved in that the contact element is further provided with at least one barbed hook for fastening the contact element in the adapter. The barbed hook is an integral part of the contact element and can be formed in a simultaneous operation during the manufacture of the contact element. When the contact element is mounted in the adapter, the contact element is introduced into the cavity of the adapter with a comparatively narrow fit. Removal, however, is not possible because the barbed hook anchored itself in the circumferential wall of the adapter. The contact element is thus fixed in the cavity of the adapter in a comparatively simple and inexpensive manner.
In an embodiment, the low-pressure gas discharge lamp is characterized in that the barbed hook is provided resiliently on the contact element. This has the advantage that the comparatively narrow fit is allowed to be somewhat less narrow and less accurate because the barbed hook is bent during insertion of the contact element and despite the less accurate fit is yet kept pressed against the circumferential wall with clamping force. The risk of rejects owing to an insufficient fit and/or damage owing to the insertion is reduced thereby, while also the risk of the contact element becoming detached from the cavity of the adapter is reduced.
In a further embodiment, the low-pressure mercury discharge lamp is characterized in that the retaining element comprises a scraping contact. A scraping contact is simple to construct, and an inserted contact pin is electrically contacted in a comparatively simple manner and securely fixed owing to its anchoring properties.
In a further embodiment, the low-pressure gas discharge lamp is characterized in that the retaining element is constructed as a resilient tongue. The resilient tongue, which is bent along with the contact pin when this contact pin is being inserted and is kept pressed against the relevant contact pin with clamping force, thus provides a permanent and good electrical contact, while at the same time the risk of the contact pin becoming detached from the retaining element is reduced. A very secure coupling between the contact pin and the retaining element is obtained when the retaining element is constructed as two resilient tongues in axially mutually opposed locations and cooperates with both contact pins of the lamp cap.
In a favorable embodiment, the low-pressure gas discharge lamp is characterized in that the contact element comprises a support wall situated axially opposite the resilient tongue for laterally supporting the contact pin on which the resilient tongue bears. The following equation holds for the compression force exerted by the resilient tongue on the contact pin:
F=C*A 
with:
F=compression force;
C=spring constant of resilient tongue;
A=deflection of resilient tongue.
When the contact pin is inserted into the retaining element, the required space for the contact pin is provided through bending of the resilient tongue. Since the resilient tongue has a deflection A which is twice the deflection of two axially mutually opposed resilient tongues, owing to the axially opposed support wall which is rigid and has substantially no deflection, a double compression force on the contact pin is realized by the resilient tongue. The risk that the contact pin may become detached from the retaining element is further reduced thereby. In addition, the advantage is obtained that the compression force is entirely generated by the contact element itself and not by the adapter, because the resilient tongue and the axially opposed support wall are integrally provided inside the contact element.
In an embodiment, the low-pressure gas discharge lamp is characterized in that the contact element has an external contact point which extends from the cavity through the bottom to the exterior of the adapter. An adapter preferably comprises 1, 2, or 4 external contact elements with external contact points. An electrical connection to other electrical elements, for example luminaires, can be realized in a simple manner by means of the external contact points.
In an embodiment, the low-pressure gas discharge lamp is characterized in that the contact element is provided with a cutting contact. A connection of, for example, an electric wiring can be realized in a simple manner by means of a cutting contact. The wiring may subsequently be guided along the lamp vessel from the one to the other end. One end of the lamp may accordingly be provided with a xe2x80x9cblind adapterxe2x80x9d. This renders it possible to provide the lamp with an electrical connection at one end only, and the lamp will be safe to touch at the same time.
In an embodiment, the low-pressure gas discharge lamp is characterized in that the adapter comprises studs in the cavity which are symmetrically positioned with respect to a mirror plane S, which extends parallel to the adapter axis and passes through the adapter axis, for supporting the contact elements and keeping them in position. Mounting of the adapter is further simplified here in that the contact elements are in a correct orientation and position before mounting.
In an embodiment, the low-pressure gas discharge lamp is characterized in that the adapter is manufactured from synthetic resin. Synthetic resin materials have the advantage over, for example, porcelain or glass that they often have a comparatively low specific mass, can be rendered at relatively low costs, and are resistant to mechanical impact.
In another embodiment, the low-pressure gas discharge lamp is characterized in that the adapter is manufactured from a ceramic material. The ceramic material, for example aluminum oxide, has the advantage over, for example synthetic resin materials, that it has a good resistance towards UV(C)-radiation and relatively high temperatures. The ceramic material has the advantage over glass that it can relatively easily be molded.
In an alternative embodiment, the low-pressure gas discharge lamp is characterized in that the adapter is provided with coupling means. Coupling means may comprise, for example, a handle or a screwthread. The lamp can thus be handled or fastened in a simple manner.
In an embodiment, the low-pressure gas discharge lamp is characterized in that a tensioning and positioning element is provided at the end of the lamp vessel. The tensioning and positioning element is provided on the end of the lamp vessel in that it is passed over the contact pins. When the tensioning and positioning element is used, a connection, for example an electric wiring, is kept under a permanent tensile stress in that the tensioning and positioning element is provided with a clamping slot through which the wiring is passed. The tensioning and positioning element further comprises a positioning passage in which a cutting contact, which forms part of the contact element and is provided in the adapter, can be placed. This renders possible a connection to the wiring simultaneous with the application of the adapter to the end of the lamp vessel, while the connection is comparatively simple.